Targeted drug delivery in which an agent (e.g., a drug or a therapeutic) is provided in an active state to a specific cell or tissue type at effective concentrations is a long sought goal. Many difficulties must be overcome to reach this goal. For instance, an agent must first be successfully delivered to the desired target. Primary delivery methods presently used include oral delivery and injections. However, injections are painful and both methods tend to provide bursts of agents rather than a preferred steady-state delivery. Additionally, the human body has developed many systems to prevent the influx of foreign substances, one of the most successful being a variety of structural components that prevent the free movement of compounds across barriers formed of epithelial tissue, e.g., the skin.
Transdermal delivery materials have been developed in an attempt to provide a painless route for delivery of active agents over a sustained period. In order to be successful, a transdermal scheme must deliver an agent across the epidermis, which has evolved with a primary function of keeping foreign substances out. The outermost layer of the epidermis, the stratum corneum, has structural stability provided by overlapping corneocytes and crosslinked keratin fibers held together by coreodesmosomes and embedded within a lipid matrix, all of which provides an excellent barrier function. Beneath the stratum corneum is the stratum granulosum, within which tight junctions are formed between keratinocytes. Tight junctions are barrier structures that include a network of transmembrane proteins embedded in adjacent plasma membranes (e.g., claudins, occludin, and junctional adhesion molecules) as well as multiple plaque proteins (e.g., ZO-1, ZO-2, ZO-3, cingulin, symplekin). Tight junctions are found in internal epithelium (e.g., the intestinal epithelium, the blood-brain barrier) as well as in the stratum granulosum of the skin. Beneath both the stratum corneum and the stratum granulosum lies the stratum spinosum. The stratum spinosum includes Langerhans cells, which are dendritic cells that may become fully functioning antigen-presenting cells and may institute an immune response and/or a foreign body response to an invading agent.
In spite of the difficulties of crossing the natural boundaries, progress has been made in attaining delivery of active agents across epithelial barriers. Unfortunately, transdermal delivery methods are presently limited to delivery of low molecular weight agents that have a moderate lipophilicity and no charge. Even upon successful crossing of the natural boundary, problems still exist with regard to maintaining the activity level of delivered agents and avoidance of foreign body and immune response.
The utilization of supplementary methods to facilitate transdermal delivery of active agents has improved this delivery route. For instance, microneedle devices have been found to be useful in transport of material into or across the skin. In general, a microneedle device includes an array of needles that may penetrate the stratum corneum of the skin and reach an underlying layer. Examples of microneedle devices have been described in U.S. Pat. No. 6,334,856 to Allen, et al. and U.S. Pat. No. 7,226,439 to Prausnitz, et al., both of which are incorporated herein by reference.
Researchers have gained understanding of the molecular world in which delivery activities occur in an attempt to improve transdermal delivery. For instance, chitosan has been found to be effective in opening tight junctions in the intestinal epithelium (see, e.g., Sapra, et al., AAPS Pharm. Sci. Tech., 10 (1), March, 2009; Kaushal, et al., Sci. Pharm., 2009; 77; 877-897). In addition, the nanotopography of a surface adjacent to a cell has been found to affect adhesive characteristics between the two as well as to effect cell behavior including morphology, motility, cytoskeleton architecture, proliferation, and differentiation (see, e.g., Hart, et al., European Cells and Materials, Vol. 10, Suppl. 2, 2005; Lim, et al., J R Soc Interface, Mar. 22, 2005, 2 (2), 97-108; Yim, et al., Biomaterials, September, 2005, 26 (26), 5405-5413). As an extension of this initial research, nanotopography of supporting substrates has been examined for use in tissue engineering (see, e.g., U.S. Patent Application Publication Nos. 2008/0026464 to Borenstein, et al. and 2008/0311172 to Schapira, et al.).
While the above describe improvements in the art, further room for improvement exists. For instance, devices and methods that provide efficient delivery of active agents across a cellular barrier including epithelial tissue would be beneficial.